Preparation of poly(trimethylene terephthalate) (3GT) polyester resins by (a) the transesterification of a C.sub.1 -C.sub.4 dialkyl ester of terephthalic acid with 1,3-propanediol, or by the esterification of terephthalic acid with 1,3-propanediol, followed by (b) polycondensation is well known in the art.
Generally, in the transesterification reaction, a C.sub.1 -C.sub.4 dialkyl ester of terephthalic acid and 1,3-propanediol are reacted in the presence of a transesterification catalyst at elevated temperature and atmospheric pressure to form bis-(3-hydroxypropyl)terephthalate monomer, along with small amounts of oligomer and C.sub.1 -C.sub.4 monoalcohol byproduct. In the esterification reaction, terephthalic acid (TPA) and 1,3-propanediol are reacted in the optional presence of an esterification catalyst at elevated temperature and at atmospheric or superatmospheric pressure to form bis-(3-hydroxypropyl)terephthalate monomer, along with small amounts of oligomer and water byproduct. The bis-(3-hydroxypropyl)terephthalate monomer and any oligomer can then be polymerized at higher temperature under reduced pressure in the presence of a polycondensation catalyst to form the desired resin.
During the process for the preparation of 3GT (transesterification, esterification and polycondensation reactions), di(1,3-propylene glycol) can be formed from intermolecular dehydration of 1,3-propanediol. This di(1,3-propylene glycol) can be incorporated into the 3GT polymer chain which affects the properties of the resulting polymer, with respect to, for example, melting temperature, glass transition temperature, crystallinity, density, dyeablity, processability, etc. The effects of the analogous impurity, diethylene glycol (DEG), on poly(ethylene terephthalate) (PET) polymer properties are well documented in the literature. For commercial grade PET the DEG levels are usually around 2-4 mol %.
Processes for the preparation of polyesters, including 3GT, have been disclosed in many patents. Some disclose use of tin and titanium catalysts.
U.S. Pat. No. 2,465,319 mentions many types of catalysts including tin. Research Disclosure 28368 (November 1987) discloses preparation of poly(alkylene 2,6-napthalenedicarboxylate) polyesters using titanium alkoxides and dibutyl tin dilaurate, etc.
U.S. Pat. No. 3,350,871 and 3,671,379, and UK Patent Specification No. 1,075,689, Example 1, show preparation of poly(trimethylene terephthalate) from dimethyl terephthalate and trimethylene glycol using a catalyst prepared by dissolving 2.5 grams of sodium in 300 ml of n-butanol, adding 37 grams of tetrabutyl titanate, and diluting to 500 ml with n-butanol. Titanium dioxide is added as a delusterant.
U.S. Pat. No. 4,166,896 describes dibutyl tin oxide as a catalyst. U.S. Pat. No. 4,611,049 describes a process for producing an aromatic polyester using an organometallic catalyst selected from the group consisting of organotitanium compounds and organotin compounds, and at least one promoter selected from the group consisting of organic sulfonic acids and aliphatic carboxylic acids. Tetrabutyl titanate, tetraisopropyl titanate, dibutyl tin oxide and butylhydroxytin oxide are preferred.
U.S. Pat. No. 5,340,909 describes preparation of poly(1,3-propylene terephthalate) using tin and titanium catalysts. Catalysts mentioned include tetrabutyl titanate, tetraisopropyl titanate, butylstannoic acid, butyltin tris (2-ethylhexoate), stannous octoate, dibutyltin tris(2-etholhexoate), stannous octoate, dibutyltin oxide and methylene bis(methyltin oxide). Tetrabutyl titanate is used in both control and demonstration examples.
U.S. Pat. No. 5,663,281 describes a process for preparing polyester polymers. At column 6 it states that (trans)esterification reactions from 1,4-butanediol using tetrabutyl titanate are satisfactory, but risk forming undesirable by-products, whereas with 1,3-propylene glycol the risk of forming undesirable by-products using tetraalkyl titanates as catalyst is not as great and, thus, "more traditional" catalysts such as tetrabutyl titanate and antimony oxide can be used. Monobutyl tin oxide is used to catalyze 1,4-butanediol reactions.
U.S. Pat. No. 5,798,433 discloses a method of synthesizing polypropylene terephthalate using 30-200 ppm titanium in the form of an inorganic esterification catalyst containing at least 50 mole % TiO.sub.2 precipitate, blocking the esterification catalyst after esterification by adding 10-100 ppm phosphorus in the form of a phosphorus-oxygen compound, and then performing precondensation and polycondensation in the presence of 100-300 ppm antimony. Table 1 shows a comparative example using titanium tetrabutylate as an esterification catalyst with antimony triacetate as a polycondensation catalyst.
U.S. Pat. No. 5,872,204 describes preparation of poly(1,3-propylene terephthalate) using ethylene glycol titanate as an esterification catalyst and polymerizing the resultant monomer in the presence of antimony acetate. At column 2 it is stated that ethylene glycol titanate does not hydrate, whereas tetrabutyl titanate does. The examples show use of ethylene glycol titanate, whereas comparative example 1 may have been directed to use of tetrabutyl titanate (compare column 12, lines 46 and 63).
None of these references mention DPG formation, specify DPG levels, nor cite the impact of DPG content on polymer end use properties, and none disclose methods to minimize DPG generation during the polymer preparation processes.
U.S. Pat. No. 5,865,424 described preparation of polyesters containing low levels of diethylene glycol wherein the reaction is carried out without a titanium catalyst.
U.S. Pat. No. 6,043,335 describes preparation of polyethylene and polybutylene terephthalates (which are stated to not have high levels of undesirable by-products) using a catalyst composition comprising a combination of a titanium-based compound, a zirconium-based compound and a phosphate-forming compound.
WO 98/23662 states that the condensation polymerization of polytrimethylene terephthalate "usually generates as much as about 4 mole percent of the bis(3-hydroxypropyl) ether which, in effect, becomes a comonomer and is incorporated into the polyester chain."
EP 1 016 692 and 1 016 741 describe polyester resin and fibers produced with no more than 2 weight % bis(3-hydroxypropyl) ether (DPG derived repeating unit). These documents describe use of metal catalysts such as titanium alkoxides (e.g., titanium tetrabutoxide or titanium tetraisopropoxide), antimony acetate or antimony trioxide. The preferred ester exchange catalysts are stated to be calcium acetate, magnesium acetate, zinc acetate and titanium acetate. In addition, they describe titanium, tin or antimony polycondensation catalysts, preferring titanium tetrabutoxide.
All of the aforementioned documents are incorporated herein by reference.